In an RFID system with a passive transponder generally the reader has two main functions. Firstly the reader supplies energy to the transponder through an RF energising field. The transponder picks this up with an antenna and resonant circuit tuned to the actuation frequency. Secondly, once the transponder is powered, the reader also needs to communicate with the remote device. These two tasks are quite different in nature and that can translate to conflicting requirements on the reader.
The powering of the transponder is important in determining the read range of the system. The transponder and reader are often weakly coupled and care must be taken to maximise the energy transfer from reader to transponder. A resonant circuit is generally used to improve the efficiency by recycling energy in the reader antenna. For maximum energy transfer, a high Q reader antenna matched to the resonant frequency of the transponder would give optimal efficiency. However, once powered, the system is required to communicate, and high Q severely limits the communication bandwidth achievable. In fact the communication bandwidth is inversely proportional to the resonance Q, therefore any improvement in the powering efficiency through an increased system Q has a direct consequence of reduced communication bandwidth.
In a half duplex system (HDX) the powering cycle and communication are separated in time. This provides flexibility in separation of the two functions of the reader and a number of approaches in the prior art have been proposed to improve the efficiency of power transfer while keeping the appropriate communication bandwidth. The most basic method is to use a separate circuit for the power and communication links, for example U.S. Pat. No. 4,550,444 and U.S. Pat. No. 4,912,471, which can therefore be optimised separately. The drawback of this approach is added complexity and cost associated with separate circuits. This is remedied by the alternative approach taken in U.S. Pat. No. 5,025,492, U.S. Pat. No. 5,374,930, and U.S. Pat. No. 5,541,604, where the same antenna is used for both power transmission and communication. A damping circuit is coupled into the resonance when the powering cycle is complete such that the powering cycle may be carried out optimally with a high Q antenna, switching to lower Q and therefore wider bandwidth for the communication cycle.
The above solution to the conflicting Q requirements is made possible because powering and reading functions are separated in time. In contrast a full duplex system (FDX) does not enable such an approach. In an FDX system the power from the reader is kept on for the duration of the read cycle. The transponder does not contain a separate transmitter, but instead communicates with the reader through modulation of the load on its pickup coil; the load modulation is picked up and interpreted by the reader. An FDX system can have the advantage of simpler transponders with lower power requirements.
Under some circumstances it can be advantageous to provide separate powering and communication antennae. This however has to be done with care as both antennae are operating simultaneously and mutual coupling can introduce problems. For example, simply setting up a powering antenna with high Q and a closely spaced communication antenna with lower Q will not necessarily offer a benefit. The modulation associated with the communication can cause ringing in the powering antenna that confuses the pickup signal in the communication antenna due to mutual coupling. The additional drawback of a multi-coil reader is increased complexity and cost.
There is therefore a need for a single antenna FDX reader that has simultaneously the properties of high Q for efficient power transfer to a transponder and also wide communication bandwidth.